Revolutionising urban mobility
Connor Lynch – Technical Business Development Manager, Giorgio Valente – Senior e-Machine and Drive Engineer, Applied Solutions Hexagon’s Manufacturing Intelligence division
Exploring eVTOL’s high power density
e-propulsion units
A deep dive into the innovative technologies underpinning the future of sustainable aviation systems.
The advent of Electric Vertical Takeoff and Landing (eVTOL) aircraft represents a groundbreaking leap in the field of aviation. These innovative vehicles promise to revolutionise urban mobility, offering an efficient and sustainable solution for short-distance travel within cities and metropolitan areas. At the heart of eVTOL's success lies the electric propulsion system (e-Propulsion unit), enabling the electric vertical flight with minimal emissions, reduced noise, and a significantly lower carbon footprint compared to traditional aviation technologies, as well as alleviating the mass of the energy storage systems which are typically battery based.
The development of these advanced propulsion units is the result of a confluence of cutting-edge technologies including high-performance electric motors, lightweight materials, advanced battery technologies, and sophisticated power electronics. When considering the numerous technical challenges associated with eVTOL, the need for high-power density e-Propulsion units becomes increasingly evident. These challenges encompass power management, thermal management, system integration, and more, making this system a focal point for engineering innovation.
The development of these advanced propulsion units is the result of a confluence of cutting-edge technologies including high-performance electric motors, lightweight materials, advanced battery technologies, and sophisticated power electronics. When considering the numerous technical challenges associated with eVTOL, the need for high-power density e-Propulsion units becomes increasingly evident. These challenges encompass power management, thermal management, system integration, and more, making this system a focal point for engineering innovation.
Hexagon’s Applied Solutions Group (ASG) have developed a 3-in-1 e-Propulsion unit concept utilising Hexagon and 3rd party software to ensure a high level of confidence in the design. The unit comprises a planetary gearbox stage mechanically coupled to a high-speed dual-redundant Permanent Magnet Synchronous Machine (PMSM) which is fed by a dual three-phase inverter. The electrical system (power electronics and e-Machine) is single-point-of-failure tolerant and is designed to provide full power in the event of loss of one of its three-phase sub-system. Thermal management of the system and lubrication of the gearbox are achieved by a single cooling circuit where oil is used to cool first the power electronics, then the e-Machine, and finally to provide adequate lubrication to the gearbox.
Figure 1. High-level e-Propulsion unit architecture.
Design approach for high reliability and functional safety
Propulsion systems are usually required to meet Design Assurance Level A (DAL-A) safety risk reduction in terms of process rigour, high reliability targets with the ability to be tolerant to a single point of failure while maintaining the overall functionality and performance required at the aircraft level, i.e. ‘Control Thrust’. Although safety is paramount, this can complicate power density endeavours.
A number of external assumptions were derived at the aircraft level for which a credible e-Propulsion unit concept could be developed. At the airframe level it was decided that there would be two independent high voltage power supply systems and two independent cooling systems. Having targeted candidate fixed-wing eCTOL and eVTOL aircraft, some additional assumptions were declared around the flight/mission profile, payload, and how many full e-Propulsion unit failures could be tolerated in order to maintain safe flight. In addition, it was considered that the e-Propulsion architecture would be ‘cross connected’ with the power and cooling systems, meaning safe flight could continue in the event of a failure in either one of these systems all be it with degraded safety margins. It was surmised that a minimum of 8 e-Propulsion units would be housed on the airframe, and that up to two independent permanent unit failures could occur without affecting safe continued flight. ). In the event of loss of 2 units, the remaining units will need to operate in overload condition. The e-Propulsion unit has been designed to withstand an overload for a maximum of 3 minutes, which is considered a sufficient time to allow the aircraft to safely land.
Using exemplar mission profiles and airframe modelling using the 1D Modelica based Elements tool, the nominal and abnormal operating propulsion performance requirements were derived. Even with aircraft-level redundancy, the preliminary safety and reliability analysis concluded that the e-Propulsion unit’s internal electrical architecture would also be required to be single-failure tolerant and provide the abnormal operating power demand for a limited time in order to enable a safe landing in the event of a hazardous failure.
The ePropulsion unit’s electronic architecture required a level of redundancy to achieve the overall DAL-A reliability target of ≤10-9/hr, unless the item was severely life limited which is clearly undesirable. It was concluded that a dual-redundant electrical architecture would be sufficient and compliant to the derived system-level safety, reliability, and availability requirements, and so a dual redundant 3-phase electrical machine concept was formed supported by two independent inverters. Supported by additional analysis in Romax, this confirmed the acceptability, in reliability terms, of a single gearbox and a single cooling system per propulsion unit.
| Parameter | Value |
| Rated output power | 100 kW |
| Overload output power | 133 kW |
| Propeller speed | 2500 rpm |
| Input HVDC voltage | 800 Vdc |
| Maximum combined efficiency | > 91% |
| Maximum altitude | 35000 ft |
| Operating temperature | -45 to 70°C |
| Target system dry mass | < 24kg |
| Maximum length |
400 mm |
| Maximum OD | 300 mm |
Table 1. e-Propulsion Unit Specifications.
Figure 2. Example eVTOL ePropulsion architecture.
Gearbox design
The housing assembly is a multifunctional component in that it supports the torque transfer from the e-machine through to the propeller, packages the power electronics, e-machine, gearbox, and drive shaft into a single unit, provides the boundary for the common electrical and mechanical cooling system and provides a simple interface with the aircraft via a flange with six mounting positions. A cross section of the complete system is shown.
The employed transmission consists of a planetary gearbox which reduces the size of the e-machine required. The gearbox has been sized to provide efficient operation and reducing the motor speed to the required speed at the propeller. The selected single planetary design was evaluated relative to a direct drive design; and a two-stage planetary design, with the single planetary design coming out most favourably on high power density.
For compactness, the sun gear has been integrated with the motor rotor shaft, and the planetary carrier upwind end supports the main bearing. This integration avoids the alignment issues of noise and wear often seen when a spline connection is used between e-machine to gearbox.
With the additional demands, the planet carrier becomes a structural member of the system, both transferring the torque from the motor to the propeller and reacting the radial and axial loads exerted on the transmission from the propeller. The planetary carrier thus becomes a complex part to design to ensure that it is suitably stiff but not over engineered and too heavy. Here, a combination of FE tools are employed to ensure that an optimum design is achieved.
Additional to the static strength requirements, the design has to consider the durability of the components for the required life of transmission. The fatigue life of the gears, bearings, and shafts are evaluated within the RomaxDT system study whilst the planetary carrier, housing and other structural parts using CAEFatigue.
For compactness, the sun gear has been integrated with the motor rotor shaft, and the planetary carrier upwind end supports the main bearing. This integration avoids the alignment issues of noise and wear often seen when a spline connection is used between e-machine to gearbox.
Additional to the static strength requirements, the design has to consider the durability of the components for the required life of transmission. The fatigue life of the gears, bearings, and shafts are evaluated within the RomaxDT system study whilst the planetary carrier, housing and other structural parts using CAEFatigue.
Figure 3. Fatigue analysis of planet carrier
e-machine design
The e-machine design consists of a dual-redundant system which is achieved by two physically separate three-phase windings. The machine topology considered for this concept design is a 12-slots/10-poles Surface-Mounted PMSM, with a Halbach magnet array which helps to further increase the magnetic loading and consequently the power density, compared to the conventional north-south arrangement. The machine cross-section with the no-load flux density map is presented in Figure 4 (right), while the dual three-phase winding schematic is shown in Figure 5 (bottom).
A concentrated winding topology has been chosen for this design because of the superior fault tolerant features when compared to the distributed winding counterpart. Indeed, each machine phase consists of an independent coil which is wound around a stator tooth so that there is no overlap or contact between different phases, neither inside the slots nor on the end windings, negating the possibility of a phase-to-phase failure.
Figure 4. Cross-section view of ePropulsion Unit.
Figure 5.1. No-load flux density map.
Figure 5.2. Radial winding pattern.
To increase reliability and reduce the risk of magnet demagnetisation at high temperatures, samarium-cobalt magnets are selected. The maximum rotor speed is 15,000 rpm, therefore high-strength carbon fibre is used to retain the magnets at a high rotational speed. The need for rotor laminations is eliminated thanks to the Halbach magnet array, while on the stator side, a cobalt iron alloy has been chosen as material for the laminations with the aim of maximising the electromagnetic performance and reducing iron losses. Thanks to the aforementioned design choices, a 97% peak efficiency is reached (see Figure 6), while the power density, calculated including both active and structural components, is 17 kW/kg.
Figure 6. e-Machine efficiency map.
Power electronics design
The design of the power electronic stage of the e-Propulsion unit poses several challenges: it must meet the strict safety requirements, while keeping a low size and weight, and it has to withstand mechanical vibration coming from the motor, gearbox and propeller. Moreover, it has to reliably operate in a hostile environment, where the impact of low air pressure imposes demanding constraints for creepage and clearance, and the effect of cosmic rays on semiconductor devices has to be considered.
The fault tolerant inverter design is achieved thanks to the selected dual redundant architecture. Separate DC link inputs and independent controllers are used for the two inverter channels. Each channel is composed of a two-level three-phase inverter where each switch is made of four SiC MOSFET devices connected in parallel. A total of six half-bridges are required for the two inverters which are physically distributed around the e-machine hexagonal housing and attached to a cold plate. An 800 Volts DC input voltage has been selected to be compatible with most of the recent battery charging infrastructures. SiC based MOSFET were chosen, as this technology drastically reduces the switching losses compared to the IGBT counterpart, and therefore allows to push the switching frequency boundaries, high switching frequency allows for control of high-speed motors (or motors with a high fundamental frequency) more effectively but leads to increased switching losses and therefore reduced inverter efficiency. A tradeoff study was then carried out to determine the optimum switching frequency showing that 20 kHz was a good compromise. The power stage has been sized to be able to deliver the full rated power in case of loss of one inverter channel (full three-phase system).
Preliminary thermal analysis has been carried out, showing a maximum operating junction temperature of 92 °C and 131 °C for healthy operation (six-phase) and faulty operations (three-phase) at the rated power, respectively. A maximum of 99.1 % efficiency can be achieved thanks to the employed state-of-the-art switches.
Preliminary thermal analysis has been carried out, showing a maximum operating junction temperature of 92 °C and 131 °C for healthy operation (six-phase) and faulty operations (three-phase) at the rated power, respectively. A maximum of 99.1 % efficiency can be achieved thanks to the employed state-of-the-art switches.
Figure 7. Mounting arrangement of half-bridges.
Figure 8.1. Half-bridge power board.
Figure 8.2. Half-bridge power board (bottom).
Thermal management and lubrication
The integrated thermal management and lubrication system includes: a single inlets; six parallel-flow channels (one for each bank of eight MOSFETs) direct-cooled motor windings; and a jet lubricated gearbox. The cooling medium is oil, chosen for both its high specific heat capacity and effectiveness as a lubricant, as well as high operating temperature range, suitable for eVTOL applications.
The switches on each power module are connected to the stator housing with a thermal interface material (TIM). The housing has a number of cooling channels which the oil passes through directly from the heat exchanger. Being first in the cooling path means that the cooling capacity of the oil is maximised. Figure 8 provides an illustration of how the power boards are distributed around the hexagonal machine housing.
To maximise thermal performance, the motor uses direct oil cooling, where the oil (after cooling the power electronics) is distributed in a circumferential channel around the centre of the motor at the outer diameter, fed radially into the slots, then passes axially through channels in the windings and out each end. There is additional flow from nozzles for the end windings and rotor, additions evidenced by the system-level 1D thermal analysis carried out on the propulsion unit. The direct cooling drastically increases the heat transfer to the oil allowing for a smaller machine whilst maintaining thermal performance.
Finally, the gearbox is lubricated and cooled by supplying oil through nozzles onto the bearings and gear flanks. Oil will be scavenged from the sump housed beneath the gearbox and filtered before passing it through a separate air-cooled heat exchanger unit and looping back to the inlet. The gearbox is last in the cooling path as it can tolerate a higher inlet temperature than the power electronics and motor.
A number of changes were implemented for the cooling system based on the results of the initial 1D thermal analysis: increasing the capacity of the heat exchanger to cope with the total heat input in the faulty overload condition; improving the switch cooling by introducing micro-fins into the cooling path to increase the heat transfer to the coolant; improve the rotor cooling by adding nozzles directed onto the end plates; and improve the winding cooling by removing the additional stator flow-path and diverting the flow through the slot cooling channels, as well as adding nozzles for the end windings. Following these changes and by increasing the flow rate effectively reduced the temperatures of the windings, MOSFETs, and magnets to within their operating limits, even after three minutes in the faulty overload condition.
A number of changes were implemented for the cooling system based on the results of the initial 1D thermal analysis: increasing the capacity of the heat exchanger to cope with the total heat input in the faulty overload condition; improving the switch cooling by introducing micro-fins into the cooling path to increase the heat transfer to the coolant; improve the rotor cooling by adding nozzles directed onto the end plates; and improve the winding cooling by removing the additional stator flow-path and diverting the flow through the slot cooling channels, as well as adding nozzles for the end windings. Following these changes and by increasing the flow rate effectively reduced the temperatures of the windings, MOSFETs, and magnets to within their operating limits, even after three minutes in the faulty overload condition.
Figure 9. Cooling and lubrication circuit.
System level performance
The 3-D model of the e-Propulsion unit is shown in Figure 10. As can be seen, the outer dimensions are 393 mm and 260 mm for the length and diameter, respectively. These are within the maximum allowable dimensions specified in Table 1. The total dry mass of the system at the current state of development is 25.8 kg, which results in a power density of 5.2 kW/kg. The mass is currently exceeding the target of 24 kg, and a design iteration with structural optimisations will be carried out to achieve the system target. It has to be noted that the power density figure is calculated for a peak power of 133 kW, whilst the system is actually sized for twice as that for redundancy.
Finally, a combined peak efficiency of about 92% could be achieved with the e-Propulsion unit, which exceeds the target set in Table 1. The efficiencies of the power electronics, e-machine and gearbox has been considered for this calculation.
Figure 10.1. Front view of ePropulsion System.
Figure 10.2. Side view of ePropulsion System.
Conclusions
Hexagon’s Applied Solutions Group set out to create a compact and efficient 3-in-1 propulsor that could play an effective part in the electric flight revolution. The electric machine developed has a specific power density exceeding the current state of the art at a high efficiency level. It has been designed to withstand both an open circuit and a short circuit fault in one of the winding systems and still deliver the overload power – meeting the requirements for DAL-A. All of this was achieved with technology which is in production today. A liquid cooling and lubrication solution is sized to enable a safe landing in the event of a fault during a mission. The mechanical system has been sized to allow the electrical system to interface with typical propellers in a lightweight and compact manner. It is directly cooled and lubricated to give the best possible durability. Overall, an efficient and safe system has been concepted that can complete the typical eVTOL and eCTOL mission profiles.
Hexagon’s Applied Solutions group are a dedicated multi-disciplinary engineering services group with over 30 years of experience delivering innovative, robust and functional designs to market across multiple industries. Utilising a CAE-led Model Based Systems Engineering approach and the entire Hexagon Portfolio.
If the specific topic of ePropulsion unit design and development is of interest, an extended version of the article has been produced and will be presented at conferences in the coming months. Hexagon’s Applied Solution Group would welcome discussion on the topic, speak to your local Hexagon sales representative to get in touch.
To find out more about the services they offer, click here.
Figure 11. Exploded view of ePropulsions System.